This invention relates to a method of preparing 1,1,1,2-tetrafluoroethane, which is useful as a refrigerant having adequate stability, by hydrogenating 1,1-dichloro-1,2,2,2-tetrafluoroethane using a palladium catalyst.
It is known that 1,1,1,2-tetrafluoroethane (hereinafter referred to as R-134a) can be obtained by fluorinating 1-chloro-2,2,2-trifluoroethane by hydrogen fluoride in the presence of a chromium oxide catalyst, but the yield of R-134a is only about 10-30%. There is a proposal of accomplishing the fluorination in a liquid phase by using potassium fluoride, but this reaction must be made under high temperature and high pressure conditions and forms potassium chloride as an inconvenient by-product. Also it is known that R-134a is obtained at fairly high yield by fluorinating trifluoroethylene by hydrogen fluoride in the presence of a chromium oxyfluoride catalyst, but trifluoroethylene is a costly material.
JP 56-38131 shows obtaining R-134a from 1,1-dichloro-1,2,2,2-tetrafluoroethane (referred to as R-114a) by reaction with hydrogen in the presence of a palladium-on-active carbon catalyst. The hydrogenation process is applicable also to 1-chloro-1,2,2,2-tetrafluoroethane (referred to as R-124), but R-124 is a costly material. In the case of the hydrogenation of R-114a the yield of R-134a reaches about 70%, but the selectivity of the reaction to R-134a is lower than 80%. That is, the reaction product includes about 10% of R-124 and about 10% of 1,1,1-trifluoroethane (referred to as R-143a). Besides, when the starting R-114a contains its isomer, 1,2-dichloro-1,1,2,2-tetrafluoroethane (referred to as R-114), by-products of the hydrogenation process include 1,1,2,2-tetrafluoroethane (referred to as R-134) and 1-chloro-1,1,2,2-tetrafluoroethane (referred as R-124a).
R-134a has a boiling point of -26.5.degree. C. When the reaction product of the hydrogenation process includes by-products having boiling points close to this boiling point, such as R-134 (b.p. -19.7.degree. C.), R-124 (b.p. -12.degree. C.) and/or R-124a (b.p. -10.2.degree. C.), difficulties are offered to the separation and purification of R-134a. For example, about 40-stage distillation towers are necessary for completely separating R-134a from R-124 by an ordinary distillation method, and by this method neither separation of R-134a from R-134 nor separation of R-124 from R-124a can be accomplished. Therefore, in the industrial practice of the hydrogenation of R-114a to R-134a there are serious problems about enhancing purity of obtained R-134a and returning by-produced R-124, which is regarded as a precursor of R-134a, to the reaction system.
As an industrial material, R-114a is usually prepared by fluorinating 1,1,2-trichloro-1,2,2-trifluoroethane (referred to as R-113) or 1,1,1-trichloro-2,2,2-trifluoroethane (referred as R-113a) with hydrogen fluoride. By this method it is inevitable that the obtained R-114a contains about 10-25% of R-114. It is very difficult to separate R-114 from R-114a by distillation since the difference between the boiling points of the respective compounds is only 0.6.degree. C.
For the vapor phase fluorination of R-113 or R-113a it is necessary to use a catalyst such as aluminum fluoride (J. Fluorine Chem, 4, 117(1974)) or chromium oxide (Chim. Ind. (Milan), 64, 135(1982)). However, aluminum fluoride is not high in the catalytic activity, and chromium oxide has toxicity and hence raises the problem about the pollution of the environment.